In medical imaging applications, two competing technologies are generally used: ultrasound and nuclear medical imaging. The benefit of the ultrasound technology is that it enables a very compact design of a probe that is powerful in revealing the anatomical structures of the organs. However, ultrasound technology is not an ideal tool in cancer detection and diagnosis because ultrasound can only generate anatomical images, whereas functional images are needed, especially at the early stages of cancer. For example, in prostate cancer diagnosis, the ultrasound probe produces and subsequently records high-frequency sound waves that bounce off the prostate's surface, and transforms the recorded sound waves into video or photographic images of the prostate gland. The probe generates images at different angles to help the physician estimate the size of the prostate and detect abnormal growths; however, benign and cancerous tumors cannot easily be distinguished by ultrasound. In addition, if the patient had radiation treatment in or around the prostate before, the fibrous tissues can be mistakenly identified as tumors during the interpretation of the sonograms. Hence, while the ultrasound probes can be designed to be very compact and easy to carry, handle, and operate, their inability to distinguish benign and cancerous tumors makes them unsuitable for functional imaging required in cancer imaging and diagnosis.
By contrast, the traditional diagnostic nuclear medical imaging techniques have the capacity to provide the desirable functional images. Such methods use radioactive tracers, short-lived isotopes that emit gamma rays from within the body and are linked to chemical compounds, permitting the characterization of specific physiological processes. The isotopes can be given by injection, inhalation, or by mouth. Normally an imaging device (e.g., Anger gamma camera as described in U.S. Pat. No. 3,011,057, which is incorporated herein by reference in its entirety) is used to image single photons emitted from an organ. The camera builds up an image of the points where radiation is emitted. This image is then enhanced by a computer, projected on a monitor, and viewed by a physician for indications of cancer. Exemplary commercial nuclear imaging systems that are capable of producing functional images include PET (Positron Emission Tomography) and SPECT (Single Photon Emission Computerized Tomography). Some of these systems are based on scintillator detectors, such as NaI, CsI and BGO, plus photon sensing devices, such as photomultipliers or photodiodes (see, e.g., U.S. Pat. No. 5,732,704, incorporated herein by reference in its entirety). Other systems are based on high-purity germanium (HPGe) crystals. Although HPGe itself is small, it needs a complex cooling system to work at cryogenic temperatures (e.g., −180° C.). Hence, all of these systems, either based on scintillator detectors or HPGe crystals, are bulky and can only be integrated into an external detection system. However, since the detectors of such external systems are located far away from the imaged organs, they have poor detection efficiency and low spatial resolution, which limit such detector's ability to pinpoint the exact positions of cancerous tissues in a small organ. All these drawbacks limit the usefulness of such radiation detection systems in diagnosing cancer in small organs, e.g. prostate glands, particularly for small tumors.
In view of the foregoing problems and drawbacks encountered in the conventional diagnostic techniques, it is highly desirable to develop a new device that would offer compact size, higher spatial resolution, and higher detection efficiency, and to provide a system with high accuracy in diagnosing cancers in small organs, e.g. prostate glands.